Add recording session timer widget

[fbraza]

Summary

Add the recording session timer that sits below the waveform.

Requirements

• Create a TimerWidget that accepts a Duration and formats it as MM:SS with leading zeros.
• Use tabular figures or monospace styling, 16-18sp font size, grey (Colors.grey[600]) color, medium weight, and letter spacing around 1.2.
• Update the timer via Timer.periodic or a streamed duration tied to the recording state.
• Reset the timer when recording stops and pause when the recording is paused.

Acceptance Criteria

• Timer displays 00:00 when idle and increments second-by-second once recording starts.
• Pausing a recording freezes the timer; stopping resets it to zero.
• Formatting maintains leading zeros and consistent character width.
• Visual styling aligns with the design guide.

[fbraza]

solved in #13